This invention relates to an air atomizing nozzle intended for use with a rewet shower for the paper making industry.
A modern paper machine produces paper from a mixture of water and fiber through consecutive processes. Three machine sections named forming, pressing and drying play the most important roles in the making of paper. Pulp at the headbox of the paper machine normally consists of about 1% fiber and 99% water.
The forming section of the paper machine removes water from the pulp by gravity and vacuum suction to form a sheet. In the pressing section, the sheet is conveyed through a series of pressing nips where additional water is removed and the fiber web is consolidated. The water concentration is reduced to about 40% after pressing. The remaining water is further evaporated and fiber bonding develops as the paper contacts a series of steam-heated cylinders in the drying section. The moisture level drops down to about 5 to 10% after the drying section.
One of the important properties of a paper product is the moisture level. However the uniformity of moisture in the paper product in both the machine direction and the cross machine direction is even more important than the absolute moisture level. There are numerous influences on the paper machine that can cause variation of the moisture content, particularly in the cross machine direction. Wet edges and characteristic moisture profiles are common occurrences on paper sheets produced by a paper machine. Therefore a number of actuator systems have been developed to offer control of the moisture profile during paper production.
One such actuator system is a water rewet shower that selectively adds small water droplets onto the paper surface. The rewet showers, which are commercially available, employ actuator nozzle units that are mounted in sequential segments (or zones) across the paper machine direction. Water flow rate is controlled independently through each actuator nozzle unit. Hence the moisture profile on the paper sheet can be adjusted by the rewet system. Spray nozzles are normally used in those rewet showers to generate droplets small enough to produce effective rewetting.
One significant component in a rewet shower is the nozzle. Droplet sizes and water mass profiles across the nozzle jets are the most important parameters to evaluate the feasibility of a particular nozzle for a rewet shower. Water particles too small tend to evaporate before they can reach the paper sheet. Droplets too big can hardly produce uniformity on the paper sheet and in extreme cases they may cause problems such as strips on the web. The ideal mass profile for the paper rewet shower generated from a single nozzle is a square shape.
The width of the square determines the zone size of the rewet shower. The height of the square represents the moisture added through this single nozzle. The coupling effects between adjacent nozzle jets are minimal in this ideal case.
Two kinds of nozzles, hydraulic and air atomizing, are widely used for water sprays. A hydraulic nozzle uses energy from a highly pressurized source to break water into droplets at the nozzle. The flow rate passing through a hydraulic nozzle is a function of the source pressure. The spraying pattern, such as spraying angle and velocity profile, is affected by the pressure as well. The fact that the droplet size is related to the flow rate makes the hydraulic nozzle ideal for operation at a fixed design point.
An air-atomizing nozzle uses energy from pressurized air to break water into small droplets. Two types of atomizing nozzle are in wide use. The internal-mixing-type nozzles mix atomizing air with water within a mixing chamber before emitting the droplet. The dependence of water flow rate on the pressure of atomizing air makes this type of nozzle unsuitable for rewet showers. The external-mixing-type nozzles mix the water with the atomizing air in an opening area outside the nozzle. The water flow rate of external-mixing-type nozzles is independent of the atomizing air pressure. The spray patterns of the external-mixing-type nozzle are affected mostly by air pressure. The droplet size from an external-mixing-type nozzle depends more on the air pressure than the water flow rate. Separating droplet size and profile controls from water flow rate control substantially simplifies the controlling strategy of a spraying system. The characteristics of the external-mixing-type nozzle make this kind of nozzle most suitable for paper rewet applications.
A simple example of an externally mixing nozzle consists of a tube surrounded by an annulus as is described by M. Zaller and M. D. Klem in xe2x80x9cCoaxial Injector Spray Characterization Using Water/Air as Simulantsxe2x80x9d, 28th JANNAF Combustion Subcommittee Meeting, CPIA Publication 573, vol. 2, pp151-160 (xe2x80x9cZaller et al.xe2x80x9d). The water flows within the tube, and the atomizing air flows in the annulus surrounding the tube in the direction parallel to the water stream. As is described in Zaller et al. this nozzle configuration can produce water droplets less than 50 microns. However the drawback of this simple nozzle is the mass profile which takes a relatively sharp peak at the center of the nozzle jet as shown in FIG. 1 by the profile labeled xe2x80x9cSingle Stream.xe2x80x9d The pulse-shaped single stream profile limits the zone size of the rewet shower.
With the same nozzle geometry as described in Zaller et al., one can introduce swirling flow in the annulus surrounding the water tube. The atomizing air moves in a direction substantially perpendicular to the water stream. German Patent No. 952,765 describes one of the xe2x80x9csingle streamxe2x80x9d nozzles that uses a swirl to break the water into droplets. The swirl generates relatively larger particles compared to the straight flow assuming that the same air pressure is employed. The drawback of the xe2x80x9csingle swirlxe2x80x9d nozzle of German Patent No. 952,765 is that the mass profile has a recess in the center aligned with the nozzle and two peaks on both sides of the recess as is shown in FIG. 1 by the profile labeled xe2x80x9cSingle Swirl.xe2x80x9d
U.S. Pat. No. 4,946,101 which is owned by the owner of German Patent No. 952,765 discloses an apparatus combining a straight stream and a swirl in the annulus surrounding the water tube. A swirling member with square threads is used to produce the required swirling flow. The combined straight and swirling flows break the water into small droplets. Centrifugal force generated from the swirl acts on water droplets and pushes them away from the center of the jet. The peak from the straight stream compensates the recess created from the swirling flow. The resulting mass profile has a relatively flat portion in the center of the jet and two relatively steep slopes on both edges as shown in FIG. 1 by the profile labeled xe2x80x9cStream-Swirl Combination.xe2x80x9d
The present invention adds to the combined straight and swirling stream another straight stream outside of and surrounding the swirling stream. One of the purposes of adding another straight stream is to add axial momentum to the particles at the outer region of the swirl which makes the slopes on the edges steeper. The resulting water profile (shown in FIG. 1 by the profile labeled xe2x80x9cStream-Swirl-Stream Combinationxe2x80x9d) created by the combination of the three atomizing air streams is closer to a square in shape than that generated from the combination of a straight stream and a swirl.
In the atomizing nozzle of the present invention a combination of three air streams is used to break the water into small droplets. A water stream with relatively low velocity is located in the center of the nozzle jet. A main air stream moving straight in the same direction as the water stream is located around the water stream. This main air stream moves much faster than the water flow inside the water stream. The shearing force generated by the large velocity gradient at the boundary of the two steams is the major force to break the water into small particles. As is described in Zaller et al. this major air stream delivers droplets less than 50 microns which is suitable for paper rewet applications. However most of the water droplets generated from this single air stream are distributed around the center of the jet. The concentrated distribution of water mass substantially limits the zone size of a rewet shower.
In order to widen the water mass profile, an air swirl that moves around the axes of both the water stream and the major air stream could be added. As is well known, the pressure outside of the swirl should be larger than the pressure inside of the swirl to maintain the circular movement of the air. The force acting on a small volume of air generated from the pressure gradient points to the center of the swirl and balances the centrifugal force acting on the same volume that points outward from the swirl""s center. Because water droplets tend to follow the air in the swirl, and the water is almost 1000 times heavier than air, the centrifugal force acting on a water droplet is about 1000 times of that of the centrifugal force acting on air occupying the same volume of the droplet.
Meanwhile the existence of water droplets in the swirl has little effect on the pressure distribution in the swirl. The outbalance between the pressure force and centrifugal force acting on a particular droplet results in a force that pushes the particle away from the swirl""s center. Adding a swirl can substantially reshape the water mass distribution. The resulting water mass distribution produced from both the major air stream and the swirl is much wider than that produced by a single major air stream as is shown in FIG. 1 by the profile labeled Stream-Swirl Combination. Although the two-stream nozzle is useful for the paper rewet application it has a drawback. The water droplet mass profile produced by a two-stream nozzle cannot be adjusted or tailored, especially at the outer edges of the profile.
The ideal water droplet mass profile of a nozzle jet for paper rewet applications is a square profile. It is the nature of a swirl that the axial momentum is weaker than the tangential momentum. Therefore the axial momentum at the outer region of the swirl is comparatively less than that in the inner region of the swirl considering there is a major air stream in the inner region. The weak axial momentum around the swirl allows water droplets to float around the swirl and never get a chance to reach the paper to be wetted. I have found that this water droplet action can be resolved by adding another straight stream outside and around the swirl. The third air stream basically pushes more water droplets at the outer region of the swirl to the paper sheet, and in combination with the swirl and the other straight stream makes the water mass distribution more like a square as shown in FIG. 1 by the profile labeled Stream-Swirl-Stream Combination.
One of the advantages of the three-stream nozzle of the present invention is to allow users to tailor the shape of the mass profile produced by the nozzle. The combination of the three streams used for atomizing purpose can be prepared and adjusted according to specific requirements on the resulting shape of the mass profile. The strength of the swirl affects mostly the width of the resulting mass profile. The inner straight stream compensates the recess in the middle of the mass profile associated with the swirling flow. The outer straight stream can be used to reshape the edges of the resulting mass profile as required.
A method of wetting webs of paper or other hygroscopic material using an atomizing nozzle. The method comprises:
(a) forming internal to the nozzle a mixed gas stream that is the combination of a gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis the another gas stream wrapping around the swirling stream and the one straight gas stream;
(b) supplying a flow of liquid into the formed gas stream so that the flow of liquid is atomized by the formed gas stream; and
(c) advancing a web of hygroscopic material across the atomized liquid flow.
A method of wetting webs of paper or other hygroscopic material. The method comprises:
(a) arranging at least first and second atomizing nozzles in an array wherein the at least first and second nozzles are adjacent to each other;
(b) forming internal to each of the at least first and second nozzles a mixed gas stream that is the combination of a gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis the another gas stream wrapping around the swirling stream and the one straight gas stream;
(c) supplying a flow of liquid into the formed gas stream so that the flow of liquid is atomized by the formed gas stream; and
(d) advancing a web of hygroscopic material across the atomized liquid flow.
A method of wetting webs of paper or other hygroscopic material using an atomizing nozzle. The method comprises:
(a) creating an array of the atomizing nozzles;
(b) forming internal to each of the nozzles a mixed gas stream that is the combination of a gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis the another gas stream wrapping around the swirling stream and the one straight gas stream;
(c) supplying a flow of liquid into the formed gas stream so that the flow of liquid is atomized by the formed gas stream; and
(d) advancing a web of hygroscopic material across the atomized liquid flow.
A method of wetting webs of paper or other hygroscopic material using an atomizing nozzle. The method comprises the steps of:
(a) creating an array of the atomizing nozzles;
(b) forming in each of the nozzles a mixed gas stream that is the combination of a gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis the another gas stream wrapping around the swirling stream and the one straight gas stream;
(c) supplying a flow of liquid into the formed gas stream so that the flow of liquid is atomized by the formed gas stream; and
(d) advancing a web of hygroscopic material across the atomized liquid flow.
An apparatus for atomizing a liquid with a gas. The apparatus comprises:
a) a housing having a gas discharging outlet and a liquid discharging outlet aligned flush with each other;
b) a first nozzle in the housing for producing at the gas discharging outlet and along a predetermined axis a mixed gas stream that is the combination of a gas stream that has a swirling movement around the predetermined axis, a first gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and a second gas stream also moving straight in the direction of the axis and wrapping around the swirling stream and the first gas stream;
c) a second nozzle disposed in the first nozzle for producing at the liquid discharging outlet a controlled stream of liquid; and
d) a gas stream divider disposed in the first nozzle and outside of the second nozzle, the gas stream divider maintaining the concentricity of the mixed gas stream and the controlled liquid stream.
An apparatus for atomizing a liquid with a gas. The apparatus comprises:
a) a first nozzle for producing in the apparatus and along a predetermined axis a mixed gas stream that is the combination of a gas stream that has a swirling movement around the predetermined axis, a first gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and a second gas stream also moving straight in the direction of the axis and wrapping around the swirling stream and the first gas stream;
b) a second nozzle disposed in the first nozzle for producing in the apparatus a controlled stream of liquid; and
c) a gas stream divider disposed in the first nozzle and outside of the second nozzle, the gas stream divider maintaining the concentricity of the mixed gas stream and the controlled liquid stream.
In a nozzle, a method for atomizing a liquid with a gas. The method comprises the steps of:
(a) forming a mixed gas stream that is the combination of a gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis the another gas stream wrapping around the swirling stream and the one straight gas stream; and
(b) supplying a flow of liquid into the formed gas stream so that the flow of liquid is atomized by the mixed gas stream.
A method for atomizing a liquid with a gas. The method comprises the steps of:
(a) forming a mixed gas stream that is the combination of a gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis the another gas stream wrapping around the swirling stream and the one straight gas stream;
(b) atomizing a flow of liquid with the formed gas stream to produce fine droplets of the liquid; and
(c) adjusting at least one of the swirling gas stream, the one gas stream and the another gas stream in the mixed gas stream so that the droplets have a predetermined mass distribution profile.
In a nozzle for atomizing a liquid with a gas, the nozzle having an outlet. The nozzle comprises:
(a) a gas stream divider for dividing a gas stream entering the nozzle into a swirling gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis; and
(b) a chamber for mixing the swirling stream, the one straight stream and the another straight stream to produce in the nozzle a mixed gas stream that is the combination of the swirling stream, the one straight gas stream and the another straight gas stream, the another straight gas stream wrapping around the swirling stream and the one straight gas stream.
An apparatus comprising:
an array of nozzles for atomizing a liquid with a gas, each of the nozzles having an outlet and each of the nozzles comprising:
(i) a gas stream divider for dividing a gas stream entering the nozzle into a gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis; and
(ii) a chamber for mixing the swirling stream, the one straight stream and the another straight stream to produce in the nozzle a mixed gas stream that is the combination of the swirling stream, the one straight gas stream and the another straight gas stream, the another straight gas stream wrapping around the swirling stream and the one straight gas stream.
An apparatus comprising:
an array of nozzles for atomizing a liquid with a gas, each of the nozzles having an outlet and each of the nozzles comprising:
(i) a gas stream divider for dividing a gas stream entering the nozzle into a gas stream that has a swirling movement about a predetermined axis, one gas stream moving straight in the direction of the axis in the inner portion of the swirling stream and another gas stream also moving straight in the direction of the axis;
(ii) a chamber for mixing the swirling stream, the one straight stream and the another straight stream to produce in the nozzle a mixed gas stream that is the combination of the swirling stream, the one straight gas stream and the another straight gas stream, the another straight gas stream wrapping around the swirling stream and the one straight gas stream; and
(iii) a flow of liquid atomized by the mixed gas stream; and a web of a hygroscopic material advancing across the array of nozzles.